Low-energy Suprathermal

Electrons in Mercury’s Magnetosphere

George C. Ho, Richard D. Starr, Jon D. Vandegriff, Stamatios M.

Krimigis, Robert E. Gold, David Schriver, Pavel Travnicek, Haje

Korth,

Ralph L. McNutt, Jr., and Sean C. Solomon

The Johns Hopkins University Applied Physics Laboratory,

Maryland, U.S.A.

MSO X

MS

O Z

M

SO

Y

Energetic Electrons at Mercury

• Energetic electrons with E>30 keV are regularly detected in MESSENGER’s orbit about Mercury.– Duration: seconds to hours,– Latitude: equator to pole,– Energy range: ≤ 300 keV,– Most intense close to the planet,– Intensity: 3 orders of magnitudes above background– Also being detected by XRS and GRNS Ho et al., Science, 2011

Low Energy Electrons Measurement• During all three Mercury flybys,

XRS observed electron-induced fluorescence

• Extensive modeling indicates they are caused by suprathermal electrons (>1 keV) interacting with the XRS detector material

P38B-04Ho et al., PSS, 2011

• Once MSRG entered orbit around Mercury, these events are routinely observed by XRS at periapsis

• The XRS dataset of these electron provide us a unique and reliable way to catalog these events

XRS

EPS

Fall AGU 2012

Algorithm to Select XRS-E Event • Developing an algorithm to select electron event using XRS

GPC data• The algorithm select event based on:

– Orbital location– Duration of an event– Intensity and Intensity difference on filtered data– Spectral slope

• Criteria were chosen to filter out:– Genuine X-ray fluorescence event– Astronomical event– Solar particle event– Electron induced planetary X-ray event (Starr et al., JGR, 2012)

Preliminary Results• 1265 events were

identified between Apr 2011 to Feb 2013 (~1700 orbits)

• Majority of the events are on the nightside

• The distribution have a peak at the dawn sector

• Majority of the events have a narrow latitudal distribution between 0° and ~40°N

• At local noon, there is a narrow band of events between 40°N to 50°N

Preliminary Results• 1265 events were

identified between Apr 2011 to Feb 2013 (~1700 orbits)

• Majority of the events are on the nightside

• The distribution have a peak at the dawn sector

• Majority of the events have a narrow latitudal distribution between 0° and ~40°N

• At local noon, there is a narrow band of events between 40°N to 50°N

Electron Energy Profiles (Schriver et al., Fall AGU 2012)Y

(Rm

)

E(keV) E(keV)

Electron energies:• bulk energies about 1-10 keV in the inner magnetosphere (XRS range)• small fraction up to ~ 50-100 keV (EPS range)

Energization due mainly to non-adiabatic motion near low magnetic field (reconnection) regions

• 1265 events were identified between Apr 2011 to Feb 2013 (~1700 orbits)

• Majority of the events are on the nightside

• The distribution have a peak at the dawn sector

• Majority of the events have a narrow latitudal distribution between 0° and ~40°N

• At local noon, there is a narrow band of events between 40°N to 50°N

Comparison with EPS ResultsXRS Events (> 1 keV) EPS Events (>35 keV)

• Both measurements show a narrow latitudal distribution at local noon

• Low energy electrons do not show the high latitude events as seen by EPS

• Different spatial distribution, or;

• Instrumental bias

Ho et al., JGR, 2012

Pitch Angle Distribution

• Like the higher energy EPS electron events, majority of the low energy XRS electron events are at pitch angle 90°

EPS Pitch Angle Distribution

Instrument Field-of-View (FoV)

Ho et al., PSS, 2011

Alexeev et al., JGR, 2008

Z (RM)

X (RM)

Discussions• The electrons are persistent at all local time with

significant bias on the dawn/night sector• The electrons are offset north of the geographic latitude

by ~20°• In particular, at local noon the electron are shifted further

upward to the north (40-60°)– Implication of the field topology

• The lack of high latitude events at lower energy could be due to:– Different energy population– Field-of-View bias of the two instruments

Summary• The XRS data provide a reliable way to identify low

energy (< 1 keV) electron events at Mercury• The electrons are persistent at all local time with

main population at ~10 keV• Preliminary results indicate the lower energy

electron have similar spatial distribution and pitch angle as the > 35keV population

• The slight difference between the two populations most likely due to field of view difference

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